Lower central series of a free associative algebra over the integers and finite fields

S. Bhupatiraju, J. Li, P. Etingof, D. Jordan, W. Kuszmaul

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Consider the free algebra A generated over Q by n generators x , . , x . Interesting objects attached to A = A are members of its lower central series, L = L (A), defined inductively by L = A, L = [A, L ], and their associated graded components = (A) defined as B = L /L . These quotients B for i ≥ 2, as well as the reduced quotient B1=A/(L2+AL3), exhibit a rich geometric structure, as shown by Feigin and Shoikhet (2007) [FS] and later authors (Dobrovolska et al., 1997 [DKM], Dobrovolska and Etingof, 2008 [DE], Arbesfeld and Jordan, 2010 [AJ], Bapat and Jordan, 2010 [BJ]).We study the same problem over the integers Z and finite fields Fp. New phenomena arise, namely, torsion in B over Z, and jumps in dimension over Fp. We describe the torsion in the reduced quotient B1 and B geometrically in terms of the De Rham cohomology of Zn. As a corollary we obtain a complete description of B1(An(Z)) and B1(An(Fp)), as well as of B2(An(Z[1/2])) and B2(An(Fp)), p > 2. We also give theoretical and experimental results for B with i > 2, formulating a number of conjectures and questions on their basis. Finally, we discuss the supercase, when some of the generators are odd and some are even, and provide some theoretical results and experimental data in this case.
Original languageEnglish
Pages (from-to)251-274
Number of pages24
JournalJournal of Algebra
Publication statusPublished - 15 Dec 2012


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